1. Field
The present specification generally relates to membrane-coated monoliths and methods for making membrane-coated monoliths and, more particularly, to cordierite membranes coated with cordierite-based composite membranes and methods for making the same.
2. Technical Background
Porous ceramic membranes are used in industrial liquid filtration separations, and have recently been investigated for gas separation and catalytic reaction. Most recently, they have been explored for gas-particulate separation in diesel particulate filter (DPF) and gasoline particulate filter (GPF) applications, and vapor-vapor separation in on-board separation of gasoline (OBS) applications. For different applications, the ceramic membrane materials and their microstructure properties must be designed differently to meet different application requirements. For example, gas-separation ceramic membranes have no pores or pore size less than 0.001 μm to separate gas molecules effectively, while DPF or GPF membranes typically require pore sizes of a few micrometers or larger to meet both low backpressure and high filtration efficiency requirements.
In some applications involving monolith substrates, it is desirable to coat a ceramic membrane on the monolith with a material such as a polymer, which may seal the pores of the monolith substrate to enable a substantially leak-free vacuum to be applied to the monolith substrate. For polymer coating processes generally, the ease of the coating process increases when the pore size of ceramic membrane being coated is minimized. To accomplish smaller pore sizes of ceramic membranes on monolithic substrates, smaller ceramic particles are required for forming the ceramic membranes. However, in some instances it can be extremely labor-intensive or even impossible to produce suitably small particles of certain materials. For example, forming a cordierite membrane with pore sizes of less than 0.3 μm is nearly impossible, owing at least in part to the general inability of conventional milling techniques to reduce cordierite particles sizes to less than 1 μm.
Though other materials such as alumina can be milled to have particle sizes less than 1 μm, pure-alumina membranes on monoliths such as cordierite are easily subject to cracking, owing to differences in the coefficients of thermal expansion between alumina and cordierite. Furthermore, coating of very small alumina particles directly onto a large pore-size monolith substrate is extremely difficult without the application of an intermediate layer.
Accordingly, ongoing needs exist for coatable ceramic-membrane materials and methods for forming ceramic membranes on monolith substrates.